Introduction
In network design, ensuring redundancy while preventing loops is crucial. The Spanning Tree Protocol (STP) plays a vital role in achieving this by creating a loop-free logical topology. One of the key concepts in STP is the Root Port, which is the switch port with the lowest path cost to the Root Bridge.
At Study4Pass, we simplify complex networking concepts to help IT professionals and students excel in certifications like CCNA, CCNP, and beyond. In this blog, we’ll explore:
- What a Root Port is
- How STP elects the Root Port
- The role of path cost in determining the Root Port
- Practical examples for better understanding
Understanding Spanning Tree Protocol (STP)
Before diving into the Root Port, let’s briefly recap STP:
What is STP?
STP is a Layer 2 protocol that prevents broadcast Cisco 200-301 Dumps storms caused by switching loops. It does this by:
- Electing a Root Bridge (the reference point for all paths).
- Blocking redundant paths while keeping backup links available.
Key STP Port Roles
- Root Port (RP) – The best path to the Root Bridge.
- Designated Port (DP) – The port that forwards traffic in a segment.
- Alternate/Blocking Port – A redundant path that is blocked to prevent loops.
Our focus today is on the Root Port.
What is the Root Port?
The Root Port is the switch port that has the lowest path cost to the Root Bridge. Every non-root bridge must select one Root Port to ensure an optimal path to the Root Bridge.
How is the Root Port Chosen?
STP uses the following criteria (in order) to select the Root Port:
- Lowest Path Cost to the Root Bridge – The port with the lowest cumulative cost wins.
- Lowest Sender Bridge ID – If path costs are equal, the switch with the lower Bridge ID is preferred.
- Lowest Sender Port ID – If Bridge IDs are the same, the port with the lower Port ID (priority + number) is chosen.
Path Cost and Its Role in Root Port Selection
What is Path Cost?
Path cost is a value assigned Cisco 200-301 Dumps Certification to each link based on its bandwidth. Lower cost = better path.
|
Link Speed |
STP Cost (IEEE 802.1D-1998) |
Revised STP Cost (IEEE 802.1D-2004) |
|
10 Mbps |
100 |
2,000,000 |
|
100 Mbps |
19 |
200,000 |
|
1 Gbps (1000 Mbps) |
4 |
20,000 |
|
10 Gbps |
2 |
2,000 |
How is the Root Port Determined?
Each switch (except the Root Bridge) calculates the total path cost to the Root Bridge. The port with the lowest cumulative cost becomes the Root Port.
Example Scenario:
Root Bridge = Switch A
Switch B has two paths to Switch A:
Path 1: 1 Gbps link (Cost = 4)
Path 2: 100 Mbps link (Cost = 19)
Which port becomes the Root Port?
- The 1 Gbps port has the lower cost (4 < 19), so it becomes the Root Port.
- Step-by-Step Root Port Election Process
Let’s break it down with a Cisco 200-301 Dumps Certification more complex topology:
Network Topology:
Root Bridge: Switch A
Switch B is connected to:
Port 1: 1 Gbps link to Switch A (Cost = 4)
Port 2: 100 Mbps link to Switch C (Cost = 19), which connects to Switch A via another 1 Gbps link (Cost = 4).
Calculating Path Costs:
Path via Port 1 (Direct to Root):
Total Cost = 4
Path via Port 2 (Through Switch C):
Switch B → Switch C (Cost = 19)
Switch C → Root Bridge (Cost = 4)
Total Cost = 19 + 4 = 23
Result:
- Port 1 has the lowest cost (4 < 23), so it becomes the Root Port.
- Port 2 will be assigned a different role (Designated or Blocking).
Common Misconceptions About Root Ports
1."Only the Root Bridge has a Root Port."
False! The Root Bridge has no Root Port—only Designated Ports.
2."A Root Port is always an uplink port."
Not necessarily. It depends on the Cisco 200-301 Dumps path cost, not physical direction.
3."All ports on the Root Bridge are Root Ports."
No! The Root Bridge has Designated Ports only.
Troubleshooting Root Port Issues
Problem: No Root Port Assigned
Cause: The switch may be isolated from the Root Bridge.
Solution: Check physical connectivity and STP configurations.
Problem: Unexpected Root Port Selection
Cause: Incorrect path costs due to manual overrides or misconfigured bandwidth.
Solution: Verify interface speeds and adjust STP costs if needed.
Conclusion
The Root Port is a fundamental concept in STP, ensuring efficient path selection toward the Root Bridge. By understanding:
- How path costs influence Root Port election
- How STP compares different paths
- How to verify and troubleshoot Root Port assignments
- You can optimize your network for stability and redundancy.
At Study4Pass, we provide simplified, exam-focused training to help you master networking topics like STP. Enroll in our courses today to boost your IT career!
- Special Discount: Offer Valid For Limited Time “200-301 Exam Dumps”
- Sample Questions for Cisco 200-301 Dumps
- Actual exam question from Cisco's 200-301 Exam
Special Discount: Offer Valid For Limited Time “200-301 Exam Dumps”
Sample Questions for Cisco 200-301 Dumps
Actual exam question from Cisco's 200-301 Exam.
Which port role is assigned to the switch port that has the lowest cost to reach the root bridge?
A) Root Port
B) Designated Port
C) Alternate Port
D) Disabled Port